The following description is provided to assist the understanding of the reader. None of the information provided or references cited is admitted to be prior art to the present invention.
The ability of T cells to recognize an antigen is dependent on the association of the antigen with either major histocompatibility complex (MHC) I or MHC II proteins. For example, cytotoxic T cells respond to an antigen that is presented in association with MHC-I proteins. Thus, a cytotoxic T cell that should kill a virus-infected cell will not kill that cell if the cell does not also express the appropriate MHC-I protein. Helper T cells recognize MHC-II proteins. Helper T cell activity depends, in general, on both the recognition of the antigen on antigen presenting cells and the presence on these cells of “self” MHC-II proteins. The requirement for recognition of an antigen in association with a self-MHC protein is called MHC restriction. MHC-I proteins are found on the surface of virtually all nucleated cells. MHC-II proteins are found on the surface of certain cells including macrophages, B cells, and dendritic cells of the spleen and Langerhans cells of the skin.
One step in mounting an immune response in mammals is the activation of CD4+ helper T-cells that recognize MHC-II restricted exogenous antigens. These antigens are captured and processed in the cellular endosomal pathway in antigen presenting cells, such as dendritic cells (DCs). In the endosome and lysosome, the antigen is processed into small antigenic peptides that are complexed onto the MHC-II in the Golgi compartment to form an antigen-MHC-II complex. This complex is expressed on the cell surface, which induces the activation of CD4+ T cells.
Other events in the induction of an effective immune response in a mammal involve the activation of CD8+ T-cells and B cells. CD8+ cells are activated when the desired protein is routed through the cell in such a manner so as to be presented on the cell surface as a processed protein, which is complexed with MHC-I antigens. B cells can interact with the antigen via their surface immunoglobulins (IgM and IgD) without the need for MHC proteins. However, the activation of the CD4+ T-cells stimulates all arms of the immune system. Upon activation, CD4+ T-cells (helper T cells) produce interleukins. These interleukins help activate the other arms of the immune system. For example, helper T cells produce interleukin-4 (IL-4) and interleukin-5 (IL-5), which help B cells produce antibodies; interleukin-2 (IL-2), which activates CD4+ and CD8+ T-cells; and gamma interferon, which activates macrophages. Since helper T-cells that recognize MHC-II restricted antigens play a central role in the activation and clonal expansion of cytotoxic T-cells, macrophages, natural killer cells and B cells, the initial event of activating the helper T cells in response to an antigen is important for the induction of an effective immune response directed against that antigen.
In addition to the roles that T cells play in the immune response, DCs are equally important. DCs are professional antigen-presenting cells having a key regulatory role in the maintenance of tolerance to self-antigens and in the activation of innate and adaptive immunity (Banchereau et al., 1998, Nature 392:245-52; Steinman et al., 2003, Annu. Rev. Immunol. 21:685-711). When DCs encounter pro-inflammatory stimuli such as microbial products, the maturation process of the cell is initiated by up-regulating cell surface expressed antigenic peptide-loaded MHC molecules and co-stimulatory molecules. Following maturation and homing to local lymph nodes, DCs establish contact with T cells by forming an immunological synapse, where the T cell receptor (TCR) and co-stimulatory molecules congregate in a central area surrounded by adhesion molecules (Dustin et al., 2000, Nat. Immunol. 1:23-9). Once activated, CD8+ T cells can autonomously proliferate for several generations and acquire cytotoxic function without further antigenic stimulation (Kaech et al., 2001, Nat. Immunol. 2:415-22; van Stipdonk et al., 2001, Nat. Immunol. 2:423-9). It has therefore been proposed that the level and duration of peptide-MHC complexes (signal 1) and co-stimulatory molecules (signal 2) provided by DCs are essential for determining the magnitude and fate of an antigen-specific T cell response (Lanzavecchia et al., 2001, Nat. Immunol. 2:487-92; Gett et al., 2003, Nat. Immunol. 4:355-60).